Welcome to Titan, the largest of Saturn’s many moons. This weird world, about the size of Mercury, is utterly alien and yet strangely Earth-like. Like Earth, Titan’s atmosphere is mostly nitrogen, but mixed with methane. It has mountains, valleys, dunes – and rivers and lakes too, but filled with an oily hydrocarbon cocktail of ethane and methane. Titan is the only other place in our neighbourhood with large bodies of liquid on its surface. And for some scientists that means the potential for life.
“We think that life requires liquid,” says NASA Ames Research Centre planetary scientist Chris McKay. Life needs a medium that brings chemicals close enough together to interact but not so close that they can’t move. Gases are too diffuse and solids too cramped to allow life as we know it to evolve. However, “there’s a bias that the liquid has to be water,” adds McKay. For good reasons. Water is an excellent solvent for the chemistry of life. It remains liquid across a wide temperature range and its polar nature helps complex molecules such as DNA and proteins to form their structure.
Most of our space exploration has revolved around the search for water – from NASA robots crawling the dusty Martian landscape to telescopes looking for distant planets that orbit their stars in the “Goldilocks zone” – not too hot and not too cold for liquid water. Water-based life is something we know how to look for. Our telescopes probe the atmospheres of planets for its key signatures – oxygen and methane.
Fly-by probes suggest huge oceans of liquid water exist deep below the frozen surfaces of Jupiter’s moon, Europa, Saturn’s moon Enceladus, and even on Titan.
All of Earth’s oxygen is generated by living organisms and 90% of its methane comes from microbes that eke out a life in watery environments, often at the extremes – in hot springs, under kilometres of ice, or in the intestines of animals. A team at NASA’s Goddard Space Flight Centre detected methane on Mars in 2003, triggering intense excitement about the its origins. In 2013 the Mars rover Curiosity found a potential methane source near Gale Crater, once a water-filled lake that microbes would happily have called home.
Fly-by probes suggest huge oceans of liquid water exist deep below the frozen surfaces of Jupiter’s moon, Europa, Saturn’s moon Enceladus, and even on Titan. The scientific consensus is that water will lead us to extra-terrestrial life forms. “Most of my biochemist colleagues are strict water chauvinists,” says McKay.
He thought this way too once.
“I got interested in life beyond Earth when Viking landed on Mars in 1976,” he recalls. “I was puzzled that there were all the indications for life but we couldn’t find any. It was like finding a house with the lights on but nobody home. That got me interested in astrobiology.”
But when Titan’s hydrocarbon seas were discovered McKay started to think differently. “For me, the particular fascination of Titan is the possibility that we might find a really alien life there – anything living in liquid methane is not related to life on Earth,” he says. DNA for instance, the backbone of life on Earth, isn’t even soluble in methane and ethane. “This life could be based on different sets of molecules and interactions,” agrees Mike Malaska,
a chemist at NASA’s Jet Propulsion Laboratory.
McKay says his search for a second genesis of life is what makes him tick as a researcher. That’s a challenge, but also an opportunity – if life were found on Titan it clearly would not have been seeded from Earth, which in turn makes it more likely life will be found elsewhere in the Universe.
Titan was spotted in the 17th century by the Dutch astronomer Christiaan Huygens. Spanish astronomer Josep Comas i Sola detected its thick atmosphere in 1907. And in 1944, Dutch-born American astronomer Gerard Kuiper (after whom the Kuiper belt is named) showed that its atmosphere contains methane and nitrogen. Nitrogen, of course, is also prominent in Earth’s atmosphere, and “so the game of pigeon-holing Titan is a rather fun one, because it is so exotic yet so Earth-like, depending on how you look at it,” says planetary scientist Jeffrey Kargel of the University of Arizona.
NASA’s Pioneer 11 probe flew past Titan in 1979, concluding it was too cold for life, followed by fly-bys from Voyagers 1 and 2 in 1980 and 1981. Despite the hazy atmosphere the Voyagers did get a few glimpses of the surface – and spotted what looked like shimmering lakes. But with surface temperatures of minus 180C the lakes could not be water. That left only hydrocarbons such as methane and ethane. NASA scientists believe solid methane was part of Titan’s original make-up and it emerged on the surface either though volcanic activity or as the result of an impact. Like the water cycle on earth, methane evaporates into the atmosphere where sunlight converts it to more complex hydrocarbons such as ethane.
The initial glimpses suggested those evaporated hydrocarbons might even have given Titan a global ocean. Among space scientists the discovery prompted many a water-cooler discussion over whether life could exist in a liquid hydrocarbon world. “Three months into my graduate career Voyager 1 flew by Titan,” says Jonathan Lunine from Cornell University. “Hydrocarbon life on Titan, if it exists, is so fundamentally different from aqueous biochemistry that one would immediately know it had an independent origin. How can one not be passionate about exploring this enigmatic world?”
In 1995 he and McKay co-authored a paper in Astrophysics and Space Science Proceedings that examined how the chemical processes that preceded life compare on Earth and Titan. Both researchers helped plan NASA’s next trip to Titan. This one would be a landing – the most distant one ever attempted in our solar system. The Cassini mission to Saturn was scheduled to launch in 1997, arrive on 1 July 2004 and continue orbiting the ringed planet. Attached to it was a probe that would separate six months into orbit. Named after Huygens, the buoyant probe was about the size of a small car and shaped like a flying saucer. It would land upon and analyse the contents of Titan’s hydrocarbon sea.
A couple of months before Huygens’ departure from Cassini, scheduled for Christmas day, a seminal paper published in Current Opinion in Chemical Biology ratcheted up the excitement about what the probe might find. Steven Benner, a prominent chemist now at the Westheimer Institute for Science and Technology in Florida, argued that although water’s polar nature helps molecules dissolve and fold in ways suitable for life on Earth, it was not a strict requirement for life.
The liquid simply had to enable molecules to react with each other – and that’s also possible in ethane and methane. “I think that it is important that we consider all liquids,” said Benner. “Titan specifically, since its seas and rivers contain the hydrocarbons that might be used to support primitive cells and primitive metabolisms.” The paper changed the discussion about methane as an alternative liquid for life, “from sort of a coffee chat to serious work,” says McKay.
Two months later Huygens parachuted down through the thick orange clouds of Titan and yielded a big surprise. It did not splash down in a methane ocean. It landed with a splat “… in methane mud,” says McKay. What happened to the shimmering oceans and lakes that Voyager had glimpsed?
Because of its limited battery life, Huygens transmitted data from Titan for only 90 minutes. It took several more months until Cassini finally spotted the first lake, and soon it became clear that while there were lakes and seas on Titan they were all clustered at the poles.
It was a setback, but in 2009 a team of NASA researchers began designing its next mission focussing on extra-terrestrial oceanography. This would be a low-cost lander called Titan Mare Explorer (TiME), targeted to Ligeia Mare, the moon’s second largest sea in the northern hemisphere. If launched in 2016 the lander would arrive on Titan in 2023, two years before the moon’s orbit takes the polar region out of direct communications contact with Earth. After that, no probe will be able to speak directly with Earth until 2040. But with NASA’s budgets tight, TiME lost out on funding to a Mars lander mission.
While McKay says there is still a chance TiME will be funded before the launch window closes, another team led by astrobiologist Nathalie Cabrol of NASA’s Ames Research Centre, together with the SETI Institute, is testing the kinds of technology a Titan lander would need. For the past three years their Planetary Lake Lander has been navigating the remote Laguna Negra in the central Andes in Chile. Granted, it is not a hydrocarbon lake, but as it’s located at the foot of the Echaurren glacier it is ideal for testing a robot that must react to both sudden and subtle changes in its surroundings.
Cassini’s instruments showed Titan’s atmosphere contains gaseous nitrogen-rich organic molecules.
After all, any Titan lander has to operate pretty much autonomously. Considering that Mars is much closer than Titan and has several orbiting spacecraft to help relay data, “we won’t be able to send as much data from Titan,” says Ralph Lorenz, a physicist at Johns Hopkins University in Baltimore, Maryland, and part of the TiME project. One part of the lake lander project is to develop on-board systems that will select the richest scientific data to send back, he says.
While Lunine and McKay wait for the next mission to Titan they try to recreate a little bit of Titan back in their labs, as they have done for more than a decade. “Life is all about building structures, capturing and using energy, and storing and transferring the information needed to synthesise the molecules to do all these things,” says Lunine. “If we can show that we have molecules on Titan that build structures and capture energy then we will have some specific predictions for what to look for when we finally do return to Titan’s surface with the next generation of instruments beyond Huygens.”
Despite arguments that Titan’s lakes would be far too cold for life to evolve, the two researchers remain undeterred. “People would say, ‘Nothing will ever dissolve in a non-polar liquid at these temperatures, give it up’,” says McKay. “The only way to address that was experimental.”
In the most recent experiments, part of a five-year NASA program, his team is testing whether a “prebiotic soup” – a solution of complex molecules from which simple life might emerge – could form on Titan. The results of the first year’s research has just been published, and has been supplemented by the latest set of data from Cassini, released in December 2013. Cassini’s instruments showed Titan’s atmosphere contains gaseous nitrogen-rich organic molecules, as well as an orange haze of solid organic particles, called tholin, which eventually falls on to the surface and into Titan’s lakes and seas.
But would tholin dissolve in the liquid – a step towards the evolution of a prebiotic soup? To make artificial tholin the team created a gas mixture of 90% nitrogen and 10% methane, and then irradiated it with simulated sunlight for several weeks. The resulting brown haze had optical properties that corresponded to those seen on Titan by Cassini. The researchers then dissolved lab-made tholin in the kinds of hydrocarbons thought to fill Titan’s lakes.
They first mixed several hundred milligrams of tholin with 100 milligrams of isopentane – a hydrocarbon from the same family as methane and ethane, which is liquid at room temperature. When the tholin dissolved they cooled the mixture to Titan temperature by diluting it with liquid ethane. Some of the tholin constituents remained in solution. “This means that solid organics from the atmosphere could indeed get into solution on the surface, making it possible for the pre-conditions for life to appear,” says McKay. “That was the biggest criticism, and we’ve successfully worked around it. We haven’t shown that life is possible, but we’ve shown that some solution is possible.”
The latest data from Cassini adds weight to the idea that McKay’s experiments really do reflect what happens on Titan’s surface. “New observations show that Titan’s lakes and rivers leave a residue when they evaporate,” says McKay. It means that the liquid in the lakes is not completely pure, but must be a solution.
Parallel to McKay’s experiments simulating Titan’s lakes, Lunine has also been conducting tests looking at sunlight. One of the main criticisms of even attempting to look for life on Titan has been its exceptionally cold temperatures. At -180°C, that’s 90 degrees colder than the coldest temperature ever recorded in Antarctica. Besides reducing the solubility of chemicals it means chemical reactions would run slowly. “Some have argued that temperatures are too low for chemical reactions to occur at speeds meaningful for the evolution of life,” says Lunine. “But life itself does not rely on chemistry driven by temperature – it relies on energy provided by sunlight through photosynthesis. So we are looking at whether something similar might occur on Titan.”
Despite the amount of sunlight on Titan’s surface being 1,000 times less intense than what reaches our planet, it’s enough for some biological processes to take place, says Lunine. The task is to identify whether any local molecules could do the job chlorophyll does in plants, capturing light energy to power chemical reactions.
One promising molecule, says Lunine, is a polymer of one of the most abundant products of atmospheric chemistry on Titan, acetylene. The researchers are now investigating whether this energy can be transferred to other molecules to initiate chemical reactions in a crude analogue of photosynthesis.
McKay and Lunine’s experiments have had a mixed reaction in the wider scientific community, although both hope that now more scientists will go beyond the “water, water, water” mantra.
But not everyone is convinced.
“The way I see it, chances are very slim that we have life on Titan and even slimmer that we know how to design an experiment whose findings on this subject we could confidently, definitively interpret,” says Carolyn Porco, planetary scientist and Cassini imaging leader.
‘I believe we know enough to say “let’s go land a mass spectrometer into the seas of Titan”.’
Still, we keep discovering new exoplanets. It is likely, says University of Arizona’s Kargel, that there are multiple Goldilocks zones out there for different liquids to be stable on a planet’s surface or in the atmosphere. “Somewhere out there is a neon moon; somewhere, an alcohol sea; somewhere, a molten magma ocean and siliceous atmosphere that rains molten rock. How many of these liquids can support life? We don’t really know,” says Kargel.
Ellen Stofan, NASA’s chief scientist, argues that “following the water” and exploring Mars, Europa and Enceladus is the first step – but “then it makes sense to push the envelope, to ask whether other liquids can facilitate life”. And the seas of Titan are the logical next target, she says.
Now that robots are becoming more sophisticated, Lunine and McKay hope NASA will decide to send a probe to Titan’s seas soon. “I believe we know enough to say ‘let’s go land a mass spectrometer into the seas of Titan’. Cassini’s mission will be over in 2017, we already know that both Enceladus and Titan are exciting places to go to, so I hope that we’re not going to wait 20 years to do so,” says McKay.
There’s another reason Titan would be an exciting place to go, says NASA’s Mike Malaska. As well as the hydrocarbons on the surface, there is a subsurface ocean of liquid water deep down under Titan’s frozen crust. “If complex organic molecules from the surface made it down to the deep ocean then you have water and complex organic molecules mixing. Then you have all sorts of exciting possibilities,” he says.
While Lunine and McKay wait for a possible future trip to Titan they keep working on their weird hydrocarbon-based chemistry in the lab. “Science is hard, it takes a long time, there are many blind alleys,” says Lunine. “To do it right takes more time than any of us reasonably have.”
But as McKay adds: “What else is there to do? Watch TV?”
Katia Moskvitch is an award-winning science and technology journalist, based in London.
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